As integrated circuit (IC) technology continues to advance, and operational clocking speeds increase, chips require more power more quickly. The traditional concept behind using capacitors to decouple ICs is to give each IC a localized reservoir of high-frequency energy. In essence, local capacitors help to “decouple” associated ICs from the main power supply, decreasing the magnitude of high frequency ripple or sag that appears on the main power bus. The bulk decoupling capacitor on a circuit board, in turn, replenishes each of the local capacitors.
Unfortunately, a capacitor is not an ideal circuit element. In fact, the capacitor is typically modeled as a series circuit, as shown in prior art FIG. 1. Here the equivalent circuit 101 for a motherboard power supply decoupling capacitor 103, a local bypass capacitor 104, and the connecting circuitry 102 between them can be seen. The power supply PS provides power to the equivalent circuit 101, which in turn passes the power on to the integrated circuit package IC. The equivalent circuit 101 may include, as modeled in this example, the mother board power supply decoupling capacitor 103 series elements CMB, ESRMB, and LMB connected in parallel with the sum of the connecting circuitry 102 series elements LPLNS+SKT and RPLNS+SKT and the local bypass capacitor 104 series elements CCPKG, ESRCPKG, and LCPKG).
Because a real-world capacitor is not ideal, including both reactive and resistive operational components, its response to transients varies as a function of frequency. Thus, at low frequencies, the capacitive reactance due to CCPKG is quite high, dominating the equivalent impedance. As the frequency increases, the capacitive reactance due to CCPKG decreases at a rate of about 20 dB/decade, while the inductive reactance due to LCPKG increases by the same amount. At the self-resonant frequency (SRF), or 1/√{square root over (LCPKGCCPKG)}, where the capacitive and inductive reactances are equal but opposite in phase, the impedance of the capacitor 104 is simply equivalent to the equivalent series resistance for the capacitor, or ESRCPKG. Above the SRF, the equivalent impedance increases, as the inductive reactance due to LCPKG dominates.
In addition, when a capacitor is placed on a circuit board, the inductance of the traces and other connecting circuitry (e.g., LPLNS+SKT) between the capacitor and the associated chip further affects chip performance at high clock speeds. The bypass element 104, and the connecting circuitry (traces) 102 that lead to it, form a current loop which operates as an antenna for transmitting radio frequency interference generated by fast transients. Thus, bypass capacitors can do their job most efficiently only if mounted in close proximity to the associated chip pins that draw transient currents.
In addition to low series inductance LCPKG in a capacitor, it's usually desirable to have a low effective series resistance (ESRCPKG), which goes hand-in-hand with a low dissipation factor. However, sometimes a very low ESRCPKG can provoke unexpected problems in the form of resonance, especially when the value of ESRMB is not matched to the sum of ESRCPKG and RPLNS+SKT and when (LPLNS+SKT+LMB)/(RPLNS+SKT+ESRMB)>>CCPKG* ESRCPKG. When repetitive pulses excite the resonator formed by a low equivalent series resistance capacitor and the motherboard, high-amplitude ringing can result, producing an exceedingly noisy supply bus. The typical solution is to place electrolytic capacitors across the bus to damp the ringing, which is costly and uses a large amount of circuit board real estate. A better solution would be to somehow increase the series resistance of the bypass capacitor ESRCPKG, without adding additional capacitance or inductance.
Thus, there is a need in the art to provide additional series resistance for bypass capacitors, connected in parallel with the equivalent series resistance of the associated circuit board power supply decoupling capacitor. Adding this type of series resistance, possibly in the form of a separate resistive element, should be accomplished at low cost, without substantially increasing the inductive reactance of the equivalent circuit. The amount of series resistance added should also be selectable, in accordance with what is necessary to dampen the resonant frequency response of the equivalent circuit between the associated power supply and integrated circuit package. There is also a need in the art for a method to add series resistance to the equivalent circuit, as described above.